1. Field of the Invention
The present invention relates to cooling tower installations, and more particularly to heat transfer systems employed in conjunction with cooling towers of the natural draft type or of the forced draft type using both indirect convective heat transfer in a so-called dry section of the installation and direct evaporative heat transfer in a so-called wet section of the installation.
2. Description of the Prior Art
A cooling tower installation of this kind is disclosed in U.S. Pat. No. 3,846,519, for example. There, the heat exchanger units of the dry section are operated only in the dry,i.e. convective, mode and the heat exchanger units of the wet section are operated only in the wet, i.e. evaporative mode. The size and number of heat exchanger units are consequently determined by the required maximum cooling output in the hot season, on the one hand, and by the need for cloud-free operation during the cold season, on the other hand. It is known that the formation of vapor clouds at low temperatures can be avoided by admixing to the moisture laden air which emerges from the wet heat transfer section of the cooling installation the warm dry air which is obtained from its dry section, after the initially cold air has been heated through convective heat transfer. In these known installations, therefore, the size of the dry section is determined by the parameters which apply to the cloud-free operation in winter, while the size of the wet section of the installation is essentially determined by the cooling output specified for the installation under extreme conditions. It follows from this that installations of this known type have to be rather large in their dimensions, having a much steeper performance curve than a cooling tower of the purely wet type.
It has also already been suggested to sprinkle water over the outer surfaces of heat exchanger units which normally are intended for convective heat transfer to the air, in order to increase the heat transfer output of these units at times when the temperature of the surrounding air is high. The water which is to be cooled still passes through the heat exchanger elements, while the latter are sprayed with either newly added water or with previously cooled water taken from the water circulation system. Another known system suggests that the water which is sprinkled over the heat exchanger elements be recirculated, being drawn from a collecting trough by means of a pump feeding it to a sprinkler system (German Offenlegungsschrift (Publ. Application) No. 2,251,709). Additional prior art suggestions can be found in the German Offenlegungsschrift No. 2,220,167 and in U.S. Pat. Nos. 2,157,070 and 2,890,864.
The units which are used for convective heat transfer between the air and the liquid cooling medium normally have finned heat exchanger pipes of a material having a high heat conductivity. These pipes are therefore either not corrosion resistant, or, if they are made of a corrosion resistant material, are very expensive. For this reason, there has recently developed a tendency to utilize heat exchanger units fabricated from plastic materials. It is well known, however, that plastic materials have a very poor heat conductivity and a comparatively low mechanical strength, so that heat exchanger units made from plastic materials require increased minimum wall thicknesses which, in turn, noticeably increase the thermal resistance across these walls. Consequently, when compared with conventional metallic heat exchanger units, the thermal resistance of plastic heat exchanger units is higher by as much as the second of third power, meaning that the heat transfer coefficient, which together with the required overall output of the system determines the total area of heat transfer surfaces needed, is correspondingly reduced.